What is time? Defining the most mysterious dimension

One of the most effective ways to define a physical quantity is to imagine the universe without it and then consider its impact when it is present. This contrast through opposites sharpens the meaning. Isaac Newton understood this intuitively. In his three laws of motion, he did not begin by directly defining force. Instead, his first law describes a universe devoid of force, where bodies move at constant velocities without interacting. Only after setting this baseline does the second law proceed to define force as the influence that causes deviation from this uniform motion, introducing an acceleration as a measure of that deviation. In essence, Newton taught us that to understand presence, we must first understand the absence.

Taking a leaf out of Newton's book, we can apply similar reasoning to time, arguably the most debated concept in physics. Imagine a universe without time. In such a universe, nothing would happen because an event implies a change from a previous state, and change requires a "before" and an "after." A universe without time would resemble a still shot that never ages. There would be no motion, no growth, and no causality. Stars would not ignite, atoms would not vibrate, hearts would not beat, and thoughts would not form. Without time, there is no development; only a static spatial arrangement that may represent a great design, but is devoid of productivity, creativity, and evolution.

With the introduction of time, sequences emerge, leading to transformations. The universe comes into existence and begins to weave the tapestry of events. Causes start to produce effects, and matter begins to organise itself into structures. Time is more than just a silent clock passively ticking in the background. It is an active enabler that sets the stage for progress, learning, and history. In this sense, time is what gives the universe its meaning, enabling and determining the pace of its evolution. Without time, the universe would be nothing more than a meaningless configuration frozen in a barren state.

Once time is defined as a deviation from a static frame, the next important question is: how do we measure this deviation? The problem we face is that time is not a tangible entity. Unlike physical matter, whose properties we can learn to a good approximation, time cannot be placed under a microscope to get a closer look at what it's made of. Our experimental tools are designed to explore physical entities such as particles, charges, fields, and waves that interact through measurable forces. However, there are no instruments capable of detecting a particle of time, nor are there colliders capable of measuring its properties.

Adding to the challenge, time's unique nature makes it behave very differently from everything we know. Matter and energy obey conservation laws. They can swap amounts between themselves or transition into one another, but their total amount remains unchanged since the universe began. In contrast, time doesn't seem to be conserved. It seems the universe perpetually generates new outputs rather than reusing existing stockpiles, preserving them as records of past events. This makes the arrow of time irreversible. While we can return to previous locations, we can't revisit past events; we can only reach them through our memories.

This peculiar temporal feature prompted us to seek alternative methods for measuring time intervals.  Since time doesn't repeat, we had to find repeating patterns to serve as ticking clocks. Sundials, some of the earliest timekeeping devices, relied on the consistent pattern of Earth's orbit. They were succeeded by pendulum clocks, which used Earth's gravity as a restoring force to maintain a steady swing. Quartz clocks measure stable frequencies of quartz vibrations, driven by an electrical current. The most advanced and precise atomic clocks measure superstable frequencies of light pulses associated with electron transitions between energy levels in atoms.

None of these clocks measures time itself. They use physical phenomena as a reference to track the natural flow. In essence, time reveals itself only through the motion of matter and the evolution of a system, never as a standalone entity. Among all physical processes, motion has proven to be the most accessible and reliable reference. It offers a precise sequence of steps that can be counted and compared with a standard. When a planet orbits, a pendulum swings, or a wave oscillates, each repetitive state marks consistent intervals. By anchoring time to motion, we can define a unified time unit by measuring displacement, oscillation, and periodicity.

The relationship between time and motion, defined by speed, reached a new level with Albert Einstein. By using the constancy of the speed of light, he expanded Newton's concepts into the realm of relativity. The measurement of time becomes reliant on the measurement of spatial coordinates, effectively merging both into a single continuum known as spacetime. To maintain the speed of light as a consistent reference, space and time units adjust by stretching and shrinking to balance each other. Yet even here, time is not measured directly. It's still inferred from physical processes, such as the behavior of particles and the movement of light through spacetime.

Remarkably, despite the abstract nature of time, our methods achieved extraordinary precision in its measurements. The artificial units we have constructed, seconds, minutes, and hours, align astonishingly well with underlying physical reality. Atomic clocks can now measure time with errors of less than a second over billions of years. This level of accuracy allows GPS satellites to synchronize across continents, spacecraft to navigate across the solar system, and high-energy experiments to test the limits of fundamental physics. Such achievements would be impossible if our operational definition of time were flawed.

This suggests a profound conclusion. Even though time itself can't be directly observed, we have managed to capture its rhythm with remarkable fidelity. By using motion as a proxy and anchoring our measurements to stable physical processes, we have built a bridge between abstract temporal flow and concrete experimental data. In doing so, we have not only learned how to measure time — we have learned how deeply it is woven into the fabric of motion, change, and the evolution of the universe itself.